The evaluation of Flow-Count and TetraONE alternative technologies for absolute CD4 and CD8 T lymphocyte determinations are being performed in five NIAID certified laboratories from different geographical locations with the Clinical Immunology Laboratory of the Children's Hospital of Philadelphia serving as the local site for these studies. Because TetraONE is a reagent/software system that operates exclusively on a Coulter EPICS XL platform, only sites using Coulter EPICS XL flow cytometers will be utilized. The five certified laboratories have been evaluating Flow-Count and TetraONE methods in a precision study measuring both the within laboratory variation and the between laboratory variation of each method and comparing the latter statistical parameters with those of the predicate method which uses flow cytometry and routine hematological counting instrumentation to determine absolute lymphocyte counts. In addition, the variation between analyses of specimens evaluated immediately (less than 6 hours old) and after standing for 24 hours at room temperature are being determined. Through December 31, 1997, five patients have been evaluated with a total of thirteen evaluations as of 2-13-98. Patient enrollment will be completed by 2-20-98, at which time biostatistical analyses of all data will begin. The results of this study will determine the application of these technologies in the measurement of other important cell subsets such as hematopoietic stem cells in peripheral blood.